The statements in this section merely provide background information related to the present disclosure and do not necessarily constitute prior art.
3D ink-jet printing technology, which is based on the operation principle of ink-jet printer, enables ink in the nozzle chamber to instantaneously form droplets under the excitation of digital signals. The droplets are ejected from the nozzle at a certain velocity and frequency, and then are cured layer by layer according to a specified path, and in the end, a 3D object is created.
Ink for 3D ink-jet printing can be categorized into light-curable ink and temperature-solidifiable ink according to different curing sources. Light-curable ink usually contains photoinitiator, oligomer and monomer, where the photoinitiator is stimulated by light irradiation to produce free radicals that promote polymerization between the oligomer and monomer to cure the ink. The temperature-solidifiable ink usually contains wax components, and the ink is solidified by lowering the ambient temperature to below the melting point of the wax. However, use of the temperature-solidifiable ink requires strict control of the temperature during the entire ink-jet printing process, and there is a risk that the printed product will melt or deform at a high temperature, and thus the temperature-solidifiable ink is limited in their application range.
On the other hand, 3D ink-jet printing ink can be classified into cyan ink, magenta ink, yellow ink, black ink, white ink and transparent ink, depending on the colorant, among which the transparent ink has always been most challenging for researchers.
The existing light-curable transparent ink are different from typical light-curable color inks such as cyan ink, magenta ink, yellow ink, black ink and white ink, the biggest drawback thereof is that the printed product tends to turn yellow and looks opaque.
More often than not, the printed products turn yellow when the light-curable transparent ink undergoes the light-curable reaction, the primary reasons are as follows: 1. the photoinitiator in ink decomposes to produce a quinone structure, which causes yellowing of the printed product; 2. the photoinitiator itself is yellow, and if the photoinitiator in the ink is not completely consumed after the light-curable reaction, then the remaining photoinitiator causes the printed product to appear yellowish; 3. free radicals produced after the photoinitiator has been irradiated with light are more reactive, and can lead to breakage of already polymerized molecular chains in the ink, resulting in a conjugated structure and thereby causing yellowing of the printed product; 4. the main reason of yellowing caused by aging of the base resin in the ink is that: conjugated double bonds produced by breakage of polymer chains, and when the amount of conjugates reaches a certain level, yellowing occurs; or carbonyl groups produced by oxidation after aging and breakage of polymer chains; or colored amines produced by decomposition of nitrogen-containing substances; 5. surfactant in the ink is incompatible with the whole system, making the printed product looks turbid.